Heightening of Dhanikhari Concrete Dam

Aimed at mitigating the shortage of potable water requirements of South Andaman area of Anadaman & Nicobar Island (India), the scheme to augment the live storage capacity of existing Dhaniknari reservoir by 3.23 mcm through raising the height of dam by 5 meters was an important project executed by NHPC Ltd. The 132 m long existing dam across the Dhanikhari nallah with dam top EL 62.5 m, consisted of eight non-overflow blocks and two spillway bays on a central overflow block of ±32 m height above deepest foundation, impounding a reservoir with gross storage capacity of 4.61 mcm at FRL 61 m. The entire water supply requirement of Port Blair is met from this reservoir.

Detail study of augmentation of the existing storage capacity of Dhanikhari reservoir, included evaluation of heightening of dam by two methods i.e. Prestressed Anchors and Backing Concrete methods. Backing concrete method was adopted due to its techno economic advantage. Several issues in this method were addressed viz. effect of permanently built-in stress on new concrete, effect of shrinkage in new concrete on old concrete, restraining effect of old concrete on new concrete, bonding between old & new concrete, enhancement of spillway capacity and seismic retrofitting, etc.

While heightening the dam, the existing overflow section was partially demolished, to construct a new spillway with augmented spillway capacity of 225 cumecs. The heightening was completed on December 2014.